Exposure to HIV and potential susceptibility to the immunodeficiency syndrome can be determined by presence of HIV-antagonizing antibodies in the blood serum of patients. A known assay for the presence of antibodies is the enzyme-linked immunosorbent assay (ELISA), which is based on an interaction between the patient's blood serum and the HIV antigen. According to the ELISA method, the inactivated virus is used as the antigen. The virus is inactivated with tensides (detergents and highly concentrated salt). ELISA microplates coated with this inactivated virus are the main component of the AIDS diagnostic tests kits produced commercially. See, Gallo et al., U.S. Pat. No. 4,520,113; Abbott Laboratories; Inst. Pasteur; Burroughs Wellcome; and others.
A major disadvantage of the ELISA test is that the inactivated virus is nonetheless considered potentially infectious. Producers of the test strictly require that the antigen (virus-coated) plates be handled as a dangerous material. Another disadvantage of the known technique resides in the fact that the virus is not readily available and is difficult to obtain in quantity for preparation of the antigen composition. The virus can be obtained only by a complex multistage technique. First, host cells must be innoculated with the virus in order to produce an infected tissue culture. The virus must then be isolated from the culture medium and purified by several successive operations, before the biological material can be used for the preparation of coated plates according to the ELISA method. This entire procedure is extremely laborious and expensive, and moreover requires the use of an extremely hazardous substrate: a virulent and contagious pathogen.
Another disadvantage of the ELISA method resides in the small but notable probability that any given test will result in a false positive response. Residual contaminants from the original tissue culture are virtually unavoidable, despite thorough purification, and some false positives are therefore inevitable. In view of the reported incidence of non-negligible falsely positive results, all positive results must be verified and confirmed by performing additional tests according to one or more different techniques, such as immunofluorescence assay or the so-called Western blot method.
These disadvantages can be eliminated or substantially diminished by the antigenic polypeptides of the invention.